This invention relates to curable polymeric compositions and to the cured composition prepared therefrom.
Due to their desirable physical and chemical properties such as toughness and solvent resistance, cross-linked acrylic and acrylic ester polymers, e.g., poly(methyl methacrylate) and copolymers, e.g., poly(styrene/ethyl acrylate), are widely employed as coating materials, glass substitutes and molding compositions.
Heretofore, several methods have been proposed for curing (cross-linking) the acrylic and acrylic ester polymers using a cross-linking agent, i.e., a polyfunctional compound reactive with said polymers which upon reaction serves as a crosslinkage between polymer molecules. For example, in U.S. Pat. No. 2,954,358, a copolymer of acrylic or methacrylic acid and an alkyl ester of one of said acids is heated at from 93.degree. to 205.degree. C. in the presence of a polyepoxide such as ethylene glycol diglycidyl ether, and optionally, a free catalyst, for a period of 30 seconds to an hour to form a cured product. Alternatively, as disclosed in U.S. Pat. No. 3,247,288, a polymerized .alpha.,.beta.-ethylenically unsaturated carboxylic acid dissolved in a polyepoxide such as the diglycidyl ether of 2,2-bis(4-hydroxyphenol)propane will cure in the presence of a catalyst in one to four hours at a temperature from 125.degree. to 200.degree. C. to form a cured composition. In addition, other acrylate polymers have been cured in the presence of an acid or base catalyst with such agents as a dicarboxylic acid; an aminotriazine; a phenolic, polyamide or urea-formaldehyde resin; or diphenylol propane diglycidyl ether. See, for example, U.S. Pat. Nos. 2,604,464; 3,020,255 and 3,058,947.
Unfortunately, the described curable acrylate polymer compositions are often unstable, i.e., the compositions cure sufficiently when stored for several days at room temperature, as evidenced by viscosity increase and gelling, to render the composition unusable. Therefore, the polymer and cross-linking agent must normally be mixed shortly before cure. Moreover, to cure the composition in a short time period, e.g., less than about one hour, relatively high temperatures are desired. In addition, following cure, the catalyst residues can migrate to the film's surface causing discoloration and loss of adhesion.
Similar problems are often experienced with self cross-linkable acrylate polymers, i.e., those polymers which can be cross-linked in the absence of a cross-linking agent. See, for example, U.S. Pat. No. 3,697,619 which discloses a curable composition of a self cross-linkable copolymer of 50-95 parts by weight methyl methacrylate and 5-50 parts by weight of the addition reaction product of an .alpha.,.beta.-ethylenically unsaturated acid and an epoxy compound.
In order to reduce the migration of the catalytic, acidic group, U.S. Pat. Nos. 3,711,449 and 4,008,293 disclose using an "internal" acid catalyst (wherein the catalytic, acidic groups are pendant to the acrylate polymer) to cross-link the acrylate polymer with a curing agent such as an aminoplast resin. Unfortunately, such curable compositions do not generally exhibit increased stability.
In view of the deficiencies of conventional curable acrylate polymer compositions and the methods for curing said compositions, it would be highly desirable to provide a curable acrylate polymer composition which is relatively stable at ambient temperatures but which cures in short periods upon the application of heat to form a hard, solvent-resistant film, coating or the like.